X-ray and Radio campaign of the Z-source GX 340+0: discovery of X-ray polarization and its implications

We present the discovery of X-ray polarization from the neutron star low-mass X-ray binary and Z-source, GX~340+0, using an Imaging X-ray Polarimetry Explorer (IXPE) observation in March 2024. Along with the IXPE observation, we conducted an extensive X-ray and radio monitoring campaign to ascertain the source properties during and around the IXPE observation. The source was within the horizontal branch throughout the multiwavelength campaign. We measured a significant X-ray polarization in 2--8 keV with polarization degree (PD) = 4. 02 0. 35% and polarization angle (PA) = 37. 6 2. 5^. The energy-dependent polarization indicates that in the 2-2. 5 keV energy range, the PA is much lower, 98^, while other energy bands are consistent with the PA found over 2. 5--8 keV. The simultaneous AstroSat-IXPE spectro-polarimetric observations provide some evidence for independent polarization from various spectral components, hinting at a disparity in the PA from the accretion disk and the Comptonized emission, while suggesting an unpolarized emission from the blackbody component. Radio observations in the 0. 7--9 GHz frequency range reveal a non-detection of radio emission in 0. 7-1. 5 GHz and a significant detection in 5. 5--9 GHz, suggesting the presence of a spectral break in 1. 5-5. 5 GHz. Using ATCA observation we place upper limits on the radio polarization at <6% on the linear polarization and <4% on the circular polarization at 3 level. We discuss the origin of the X-ray polarization and its implications on the geometry of the spectral components.